1. Field of the Invention
The present invention relates to image heating apparatuses that heat toner images on sheets. Such image heating apparatuses may be used in image forming apparatuses such as, for example, copiers, printers, facsimile machines, and multi-function machines equipped with a plurality of functions of these devices.
2. Description of the Related Art
Electrophotographic image forming apparatuses perform an electrophotographic process in which toner images are fixed on recording paper (sheets) through charging, exposing, developing, and fixing steps. As an example of fixing apparatuses (image heating apparatuses) that perform the fixing step, in Japanese Patent Laid-Open No. 2010-26489, a fixing apparatus to which a fixing belt method is applied is proposed. With this fixing apparatus, heat capacity is decreased by using a thin fixing belt (endless belt).
In the fixing apparatus described in Japanese Patent Laid-Open No. 2010-26489, a nip portion is formed by the fixing belt and a pressure roller, and a pressure pad that presses the fixing belt in a direction from the inner surface of the fixing belt toward the pressure roller is provided.
In such a fixing apparatus, the inner surface of the fixing belt and the pressure pad are in a relationship in which the fixing belt slides against the pressure pad as the fixing belt is rotated, and accordingly, the inner surface of the fixing belt and the pressure pad tend to wear off.
In order to address this, in the fixing apparatus described in Japanese Patent Laid-Open No. 2010-26489, lubricant (fluorine-based grease) is applied between the inner surface of the fixing belt and the pressure pad, and layers formed of a fluorine-based resin are provided on the inner surface of the fixing belt and the pressure pad.
However, nowadays, with increasing demand for fixing apparatuses with which operational speed and image quality are increased, the pressure at the nip portion is desirably set to be higher than that of the related art. In this regard, there is a concern as described below regarding the structure of the fixing apparatus described in Japanese Patent Laid-Open No. 2010-26489.
That is, the fluorine-based resin layers coated on the inner surface of the fixing belt and the pressure pad may significantly wear off due to increased pressure at the nip portion, and accordingly, torque for rotating the fixing belt may be increased. As a result, a self-induced vibration referred to as stick-slip occurs in a portion where the fixing belt slides against the pressure pad. When the degree of stick-slip becomes non-negligible, the user may feel uncomfortable with noise caused by stick-slip (such a phenomenon is referred to as “squeal” herein).
This phenomenon is caused by the fact that particles of the fluorine-based resin, which are produced as a result of the fluorine-based resin layers sliding against each other and being rubbed off, contribute to an effect by which wear on the fluorine-based resin is decreased. When considering use of the fixing apparatus for a long time in view of the above-described characteristics, the thicknesses of the fluorine-based resin layers needs to be significantly increased. This is not a realistic measure.
Furthermore, lubricant, which is present in a portion where the fixing belt slides against the pressure pad, is repelled due to the characteristics of the fluorine-based resin with which the fixing belt and the pressure pad are coated, thereby degrading the lubricating effects produced by the lubricant.
As described above, even when suppression of wear caused by sliding of the fixing belt against the pressure pad is attempted by coating the fixing belt and the pressure pad with a fluorine-based resin, such a measure is not sufficiently effective.